To assess the potential of monoclonal antibodies that inhibit platelet function in vitro as in vivo therapeutic agents, F(ab')2 fragments (0.17 to 0.81 mg/kg) of a murine monoclonal antibody (7E3) that binds to platelet glycoproteins IIb and/or IIIa and blocks platelet aggregation induced by ADP were infused into three dogs. Soon after infusion, platelets recovered from the dogs showed a decreased aggregation response to adenosine diphosphate, with the highest dose producing nearly total inhibition. These platelets also showed decreased ability to bind 125I-7E3, which was assumed to reflect occupancy of the sites by the unlabeled F(ab')2 fragments. At the highest dose, the binding decreased by 85%, reflecting the binding of approximately 44,000 molecules of 7E3 F(ab')2 per platelet. Platelet counts decreased after antibody infusion by less than 20%, and none of the dogs showed spontaneous bleeding. Both the aggregation and binding results reverted toward normal within one day. We conclude that it is possible to profoundly inhibit platelet function by in vivo infusion of F(ab')2 fragments of a monoclonal antiplatelet antibody without producing spontaneous hemorrhage or significant thrombocytopenia.